The present invention relates to an apparatus for regulation during laser beam welding.
For some years, lasers have been used in industrial production, particularly for welding, cutting, and surface treatment. In the automotive industry, for example, laser welding technology is increasingly gaining importance because of the high processing speeds which can be achieved, the low thermal stress on the workpiece, and the high degree of automation which is possible. Connected with the use of this technology is the need for quality assurance of the weld seam, which has been produced, on the basis of the possibility of monitoring (and regulating) the welding process.
From "Opto-elektronischer Sensor fur die Echtzeitbeobachtung beim Laserschwei.beta.en zur Nahtfuhrung und adaptiven Prozessbeeinflussung" (Opto-electronic sensor for real-time observation during laser welding for seam guidance and adaptive process influence) by W. Juptner and B. Hollermann, Laser und Optoelektronik (Lasers and opto-electronics), 1990, 56 ff., a video-optic sensor is known in which image-assessing electronics are used to determine characteristics of the melt bath, and which, for photographing the melt bath geometry, comprises a black-and-white CCD camera for photographing a melt bath of a laser welding process, which bath is additionally illuminated with an illumination laser. The images of a melt bath, which are obtained, show the disruptive influence of the glow of the plasma produced in the center of the welding process, which effect is to be suppressed using an intensively radiating infrared lamp. In order to be able to assess the melt bath geometry, the image obtained is highly filtered (by using a Laplace filter and/or a low-pass filter and/or a Sobel filter) and binarized, as well as Fourier-transformed, if necessary. In the image obtained in this way, the melt bath cannot be shown in its entirety, since because of the prior image processing, the bright region obtained reproduces only parts of the melt bath, but not the real melt bath geometry, so that an assessment of an individual image cannot result in a clear statement about the geometry of the melt bath edge. For this reason, the inclusion of ten or more photographs by means of successive superimpositions is required in the known system.
From "Thermografische Bilderzeugung und -verarbeitung beim Laserschwei.beta.en" (Thermographic image generation and processing during laser welding) by G. Bruggemann in: Bildhafte Darstellung und Auswertung der Ergebnisse der ZfP (Pictorial representation and assessment of the results of nondestructive testing), DGZfP--Deutsche Gesellschaft fur zerstorungsfreie Prufung (German Society for Nondestructive Testing), Stutensee, Nov. 27-28, 1995, a system for quality control during laser welding is known, which comprises a CCD camera with polarization filters and metal interference filters set in front of it for selective and partial attenuation, by means of which the welding process is photographed in near infrared region. Since the heat radiation is already darkened to such an extent after the selective filtering of the light radiating from the melt bath, that further global darkening would cause the information about the heat field to be false, only local image fragments of the laser plasma and its direct surroundings are partially attenuated (darkened) by means of a gray filter mounted directly on the camera chip. Since a quantitative temperature measurement is to be conducted with the known system, a temperature is assigned to each gray value of the CCD camera. Further assessment is based on a comparison with reference geometries.
Furthermore, from "Analyse thermographischer Bilddaten zur On-Line-Uberwachung von Laserstrahlprozessen" (Analysis of thermographic image data for on-line monitoring of laser beam processes) by G. Bruggemann and F. Heindorfer in: Schwei.beta.en und Schneiden (Welding and cutting), 1994, 622-625, a system for monitoring welding processes is known, in which, by recording the temperature field which is emitted during the welding process, the melt bath, i.e. its geometrical dimensions permit conclusions to be drawn with regard to changes in laser output and/or feed velocity, splitting and offset problems, through-welding losses, or the like.
From U.S. Pat. No. 5,517,420, an apparatus for regulating welding parameters during laser beam welding is known, in which the geometry of an interaction zone formed during the welding process is photographed using a camera, particularly a CCD camera, the camera being connected with an image-processing unit. From the image taken with the camera, spatial information about the size of the interaction zone is obtained by determining the number of bright pixels exceeding a predetermined number, the interaction zone comprising the melt bath and the welding plasma superimposed on the melt bath. This information is processed using a so-called fuzzy logic control and serves as the basis for stable control of the welding speed.
Proceeding from this state of the art, the invention is based on the task of making available an apparatus for regulating welding parameters during laser beam welding, in which direct on-line regulation of the laser welding process, particularly the depth and the location of the welding seam, and therefore minimization of welding defects during a welding process, is made possible.
Pursuant to the invention, to accomplish this task, an apparatus for regulating welding parameters during laser beam welding is proposed for regulating welding parameters during laser beam welding, comprising a CCD camera for detecting the geometry of a melt bath formed during the welding process, and an image data processing unit, wherein the camera is operably connected with the image-data-processing unit such that the welding depth is regulated as a function of the melt bath length or melt bath area detected.
Accordingly, the welding depth of the laser beam is regulated as a function of the detected melt bath length or melt bath area. The parameter of melt bath length or melt bath area can be derived directly and easily from the image of the melt bath detected by the camera. Studies have shown that the welding depth of the laser beam in the workpiece to be welded is related linearly to the length or area of the melt bath, so that the detected parameter of melt bath length represents a direct measure of the welding depth. This relationship is utilized pursuant to the invention for regulating the welding depth during the welding process, so that complicated temperature value assignment and calculation of a temperature profile of the melt bath is no longer necessary.
Furthermore, objects of the invention are accomplished by an apparatus for regulating welding parameters during laser beam welding comprising a CCD camera for detecting the geometry of a melt bath formed during the welding process, and an image processing unit wherein the camera is operably connected with an image-data-processing unit such that the focus position of the laser beam is regulated as a function of a geometric similarity factor, which is calculated as the quotient of the melt bath area and the distance between the geometric centers of gravity of the laser beam keyhole and the melt bath area. Accordingly, the focus position of the laser beam is regulated as a function of a geometric similarity factor, without the need for any temperature value assignment or calculations. The geometric similarity factor represents an empirical value of the focus position and is calculated as the quotient of the melt bath area and the distance between the geometric centers of gravity of the laser beam keyhole and the melt bath area. The geometric similarity factor includes the thought that a change in the focus position from the zero position will result in a change in the melt bath geometry.
As a development of the invention, a gap offset and/or the seam location of the laser beam is regulated as a function of the detected geometry of the melt bath front. The occurrence of a gap, a height offset, or an offset of the seam location of the laser beam results in a change in the geometry of the melt bath front in each instance, which can be detected by the inventive apparatus. Pursuant to the invention, the possibility of direct detection and regulation of a gap offset (for example a height offset at the zero gap or the existence of a gap with a small width) and/or the seam location of the laser beam is made available therewith.
As a development of the invention, recognition of melt bath ejections takes place via detection of the melt bath length or melt bath area. Melt bath ejections, which occur as the result of welding defects, result in a sudden change in the melt bath geometry, which is detected pursuant to the invention as a collapse of the melt bath length or melt bath area, so that direct and immediate recognition of melt bath ejections, which cause weld defects, is made possible.
In an advantageous development of the invention, a gray value assignment of the pixels of the melt bath image detected by the camera takes place, advantageously, a two-stage binarization with two gray-value limits being involved. Compared with pure binarization (white and black), the detected image of the melt bath is divided by this method into three regions, namely a region for the so-called keyhole of the laser beam, a region for the actual melt bath, as well as a region, which reproduces the surroundings of the melt bath. This inventive procedure for image processing yields an easily reproducible representation of a melt bath with clearly defined contours, which can be used as the basis for further inventive assessment and regulation.